Ubuntu Manpage: CURLINFO_TLS_SESSION, CURLINFO_TLS_SSL_PTR

Provided by: libcurl4-doc_7.81.0-1ubuntu1.20_all

NAME

       CURLINFO_TLS_SESSION, CURLINFO_TLS_SSL_PTR - get TLS session info

SYNOPSIS

       #include <curl/curl.h>

       CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_TLS_SSL_PTR,
                                  struct curl_tlssessioninfo **session);

       /* if you need compatibility with libcurl < 7.48.0 use
          CURLINFO_TLS_SESSION instead: */

       CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_TLS_SESSION,
                                  struct curl_tlssessioninfo **session);

DESCRIPTION

       Pass  a  pointer  to  a  'struct  curl_tlssessioninfo  *'.  The pointer will be initialized to refer to a
       'struct curl_tlssessioninfo *' that will contain  an  enum  indicating  the  SSL  library  used  for  the
       handshake and a pointer to the respective internal TLS session structure of this underlying SSL library.

       This  option  may  be  useful  for  example to extract certificate information in a format convenient for
       further processing, such as manual validation. Refer to the LIMITATIONS section.

       struct curl_tlssessioninfo {
         curl_sslbackend backend;
         void *internals;
       };

       The backend struct member is one of the defines in the CURLSSLBACKEND_* series: CURLSSLBACKEND_NONE (when
       built     without     TLS     support),      CURLSSLBACKEND_WOLFSSL,      CURLSSLBACKEND_SECURETRANSPORT,
       CURLSSLBACKEND_GNUTLS,       CURLSSLBACKEND_GSKIT,       CURLSSLBACKEND_MBEDTLS,      CURLSSLBACKEND_NSS,
       CURLSSLBACKEND_OPENSSL, CURLSSLBACKEND_SCHANNEL or CURLSSLBACKEND_MESALINK. (Note that the OpenSSL  forks
       are all reported as just OpenSSL here.)

       The  internals  struct  member will point to a TLS library specific pointer for the active ("in use") SSL
       connection, with the following underlying types:

              GnuTLS gnutls_session_t

              gskit  gsk_handle

              NSS    PRFileDesc *

              OpenSSL
                     CURLINFO_TLS_SESSION: SSL_CTX *

                     CURLINFO_TLS_SSL_PTR: SSL *
       Since 7.48.0 the internals member can point to these other SSL backends as well:

              mbedTLS
                     mbedtls_ssl_context *

              Secure Channel
                     CtxtHandle *

              Secure Transport
                     SSLContext *

              wolfSSL
                     SSL *

              MesaLink
                     SSL *

       If the internals pointer is NULL then either the SSL backend is not supported, an SSL session has not yet
       been established or the connection is no longer associated with the easy handle (eg curl_easy_perform has
       returned).

LIMITATIONS

This option has some limitations that could make it unsafe when it comes to the manual verification of
certificates.

This option only retrieves the first in-use SSL session pointer for your easy handle, however your easy
handle may have more than one in-use SSL session if using FTP over SSL. That is because the FTP protocol
has a control channel and a data channel and one or both may be over SSL. Currently there is no way to
retrieve a second in-use SSL session associated with an easy handle.

This option has not been thoroughly tested with plaintext protocols that can be upgraded/downgraded
to/from SSL: FTP, SMTP, POP3, IMAP when used with CURLOPT_USE_SSL(3). Though you will be able to retrieve
the SSL pointer, it's possible that before you can do that data (including auth) may have already been
sent over a connection after it was upgraded.

Renegotiation. If unsafe renegotiation or renegotiation in a way that the certificate is allowed to
change is allowed by your SSL library this may occur and the certificate may change, and data may
continue to be sent or received after renegotiation but before you are able to get the (possibly) changed
SSL pointer, with the (possibly) changed certificate information.

If you are using OpenSSL or wolfSSL then CURLOPT_SSL_CTX_FUNCTION(3) can be used to set a certificate
verification callback in the CTX. That is safer than using this option to poll for certificate changes
and does not suffer from any of the problems above. There is currently no way in libcurl to set a
verification callback for the other SSL backends.

How are you using this option? Are you affected by any of these limitations? Please let us know by
making a comment at https://github.com/curl/curl/issues/685

PROTOCOLS

       All TLS-based

EXAMPLE

       #include <curl/curl.h>
       #include <openssl/ssl.h>

       CURL *curl;
       static size_t wf(void *ptr, size_t size, size_t nmemb, void *stream)
       {
         const struct curl_tlssessioninfo *info = NULL;
         CURLcode res = curl_easy_getinfo(curl, CURLINFO_TLS_SSL_PTR, &info);
         if(info && !res) {
           if(CURLSSLBACKEND_OPENSSL == info->backend) {
              printf("OpenSSL ver. %s\n", SSL_get_version((SSL*)info->internals));
           }
         }
         return size * nmemb;
       }

       int main(int argc, char** argv)
       {
         CURLcode res;
         curl = curl_easy_init();
         if(curl) {
           curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
           curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, wf);
           res = curl_easy_perform(curl);
           curl_easy_cleanup(curl);
         }
         return res;
       }

AVAILABILITY

       Added in 7.48.0.

       This option supersedes CURLINFO_TLS_SESSION(3) which was added in 7.34.0.  This  option  is  exactly  the
       same as that option except in the case of OpenSSL.

RETURN VALUE

       Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.